Cytochrome P450 CYP2D6 belongs to the CYP2 family of P450s and is the only functionally active isozyme of the CYP2D subfamily in humans. It is involved in the metabolism of up to 25% of all therapeutically used drugs (Hardman et al, 1995). The gene encoding its synthesis is located in the CYP2D locus at q13.1 on the long arm of chromosome 22 (Eichelbaum et al., 1987). It is part of a gene cluster containing also two pseudogenes CYP2D7P and CYP2D8P (Kimura et al., 1989). Like other members of the human CYP2 gene family, the CYP2D genes consist of 9 exons and 8 introns. The enzyme exhibits a common genetic polymorphism (Meyer and Zanger, 1997). In fact it was the first cytochrome P450 enzyme for which a genetic polymorphism was demonstrated which was named the debrisoquine/sparteine polymorphism based on the two substrates involved in its discovery (Mahgoub et al., 1977; Eichelbaum et al., 1979). Depending on the metabolic handling of these two probe drugs, between 5 and 10% of subjects of European populations have a severely impaired capacity to form the major metabolites 4-hydroxydebrisoquine and 2-dehydrosparteine. These subjects were designated as poor metabolizers (PM), the remainder of the population being so-called extensive metabolizers (EM). The trait ‘poor metabolism’ is inherited in an autosomal recessive fashion, i.e. PMs are carriers of two non-functional alleles. The molecular basis of this polymorphism has been extensively investigated and more than 30 functional and non-functional alleles have been described which allow to predict the PM phenotype in Caucasians with an estimated 99% reliability. (Daly et al., 1996 and CYP Allele Nomenclature Web-Site: http://www.imm.ki.se/CYPalleles/cyp2d6.htm).
Over 50-fold variability in CYP2D6 activity exists among extensive metabolizers (genetically carriers of at least one functional allele) which has led to the designation of the most rapid “extensive” phenotype as “ultrarapid” (UM) and the slowest as “intermediate” metabolizer (IM). There is evidence in the literature that these sub-phenotypes are clinically relevant. Individuals with the UM phenotype are at risk to experience therapeutic failure due to abnormally fast clearance of the drug whereas IMs may be comparable to PMs in their risk to develop adverse side effects and toxicity.
A molecular explanation for the UM phenotype was provided by the discovery of the CYP2D6 gene duplication, which however only applies to a fraction of UMs (Johansson et al., 1993; Dahl et al., 1995). Two CYP2D6 alleles previously described result in lower enzyme activity and cause the IM phenotype in individuals not carrying a normal functional allele. However, both of these alleles (*9: Broly and Meyer, 1993; *10: Yokota et al., 1993) occur with a frequency of only 2% in the Caucasian population and only about 20% of the IMs have informative genotypes involving these two alleles (i.e. *9/*0, *10/*0 and *10/*10; Sachse et al., 1997; Griese et al., 1998). 80% of the IMs have therefore “uninformative” genotypes, i.e. genotypes that are also associated with the normal extensive or the ultrarapid metabolizer phenotypes. It has therefore remained unclear whether the IM sub-phenotype has a genetic basis or whether it is an epigenetic phenomenon.
It is clear that naturally occurring mutations, if they exist can have effects on drug metabolization and efficacy of therapies with drugs. It is unknown, however, how many of such variations exist, and with what frequency and at what positions in the human CYP2D6 genes.
Accordingly, means and methods for diagnosing and treating a variety of forms of individual drug intolerability and inefficacy of drug therapy which result from CYP2D6 gene polymorphisms that interfere e.g., with chemotherapeutic treatment of diseases was hitherto not sufficiently available but are nevertheless highly desirable.
Thus, the technical problem of the present invention is to comply with the needs described above.
The solution to this technical problem is achieved by providing the embodiments characterized in the claims.